Letters from Grand Canyon

Sixty-five million years ago, the Mesozoic era came to an end. With it went the great reptiles that once lorded it over land, sea and air, victims perhaps of unimaginable catastrophe when an asteroid smashed into Earth near Yucatan's Chixculub, visiting death and destruction upon most living things. With the reptiles gone, the coast was clear for the mammals to develop explosively, leading eventually to the arrival of Homo sapiens, that strange bipedal primate whose most notable characteristic is to be often clever and seldom wise.
Changes were also brewing for the area later to become the Colorado Plateau of the western United States, changes that, while not so fiery, were also to have profound consequences. As we saw in the previous Letter, the transition from the Mesozoic to the following Tertiary left the Rocky Mountain West with a landscape which, though quite different from what we see today, would evolve gradually but steadily into the present scene.
In the West, in what are now parts of Utah, Nevada and Arizona, a great mountain chain—still being uplifted at that time—spilled vast aprons of debris eastward and northward onto what is now the Colorado Plateau (Figure 1). To the east of the mountain chain, and extending roughly north-south in this area, was a seaway (the “Interior Seaway” of the last Letter.) Between mountain and sea, an enormous alluvial plain was crossed by rivers flowing eastward from the former to the latter. It is quite possible that a reasonably accurate analog (without the sea, of course) would be today's interface between the Rocky Mountains and the High Plains: a mountain front cut by deep canyons that spill onto nearly featureless plains.
This landscape cannot be seen today because it has been destroyed by subsequent events. It can only be reconstructed in the mind's eye using what evidence we have: marine deposits left by the Cretaceous “Interior Seaway”; fluvial deposits left by rivers on the alluvial plains; the sheets of debris derived from the mountains (as shown by the debris' composition and its coarsening toward the mountains); and intense deformation along the old mountain belt.
Actually, it is not entirely correct to say that the old mountains are no longer visible. As you travel south from Flagstaff to Phoenix on Interstate 17 you come to a viewpoint (at mile marker 313) just as you leave the Mogollon Rim from which you look south into the Verde Valley and the Black Mountains and other ranges beyond in central Arizona. This is where the ancient mountain belt used to be, torn apart and foundered long ago as a result of basin-range pull-apart deformation that produced basins such as the Verde Valley. In spite of the foundering, traces of the old mountains and their loftiness remain, because the central Arizona mountains, which even today are at least as high as the edge of the Colorado Plateau where the viewpoint is located, are composed of Precambrian rocks, whereas the edge of the Plateau is capped by uppermost Paleozoic rocks. These Paleozoic rocks have been eroded from the central Arizona ranges; restoring the rocks would add thousands of feet to the mountains, making them much higher than the Plateau even after the foundering. A similar situation is visible on Interstate 40 west of Seligman, where ranges such the Hualapai and Peacock Mountains, which are composed of Precambrian rocks and are beyond the Plateau's edge where the ancient mountain belt used to be, still tower above the Plateau's edge in spite of faulting and foundering.
So long as the simple picture of mountain/alluvial plain/sea existed, the drainage network maintained an equally simple configuration: from mountain to sea, meaning generally eastward or northeastward in Utah, and northward to northeastward in northern Arizona. But the simplicity was spoiled by continuing deformation, which produced additional mountains and hills where before there had been alluvial plains and sea. The result was that the sea disappeared about 65 million years ago. By about 55 million years, the Rocky Mountain West started to split into individual basins, each rimmed by mountains, each occupied by a lake. In each lake, sediments accumulated that today bear picturesque names such as Wind River Formation, Green River Formation, Uinta Formation—all testimony to the basins where the lakes once held sway, where these rocks are to be found today, and which once were dear to native man and mountain man alike.

Notable among the uplifts of this time are those in Colorado and northern New Mexico, roughly where the Rocky Mountains and San Juan Mountains are now. These uplifts introduced westerly slopes from mountain to lake, instead of just the easterly slopes that had been the norm earlier. The consequence was that now there were many drainage basins, each consisting of streams flowing from all directions into the lake that occupied the low part of the basin (Figure 2). One of the largest lakes—Lake Flagstaff and then Lake Uinta—occupied eastern Utah, northwest Colorado and southwest Wyoming, and persisted until about 40 million years ago. This lake was the sump for a very large area athwart the present course of the Colorado River. Presumably, it also drained the southwest Colorado Plateau in northern Arizona, but we have no direct evidence of this because rocks of that age were never deposited or eroded after deposition. This much is known: coarse deposits derived from the ancient mountains south of the present Plateau rim continued to be deposited until Oligocene time, 25 to 37 million years ago, indicating drainage from south to north onto the Plateau. These deposits are known as the “Rim gravels.” Farther west, in the Peach Springs/ Kingman area, volcanic material flowed northeastward onto what is now the Plateau as recently as 18–20 million years ago, indicating persistence of that drainage direction. Near Fraziers Well, on the road to Havasu, Eocene limestone and gravel rest directly on the Paleozoic Kaibab Limestone, showing that the entire sequence of Mesozoic rocks had been eroded from that area by 37–55 million years ago. However, it is possible that many of the Mesozoic sediments had never been deposited so far south, or were much thinner than to the north. It is also true that the Moenkopi Formation, the lowest and oldest Mesozoic rock of the area, was present at Red Butte, south of the Grand Canyon, about 14–15 million years ago; directly north of the Grand Canyon on the Shivwits Plateau six to seven million years ago; and southeast of Flagstaff four to five million years ago. Evidently, Mesozoic rocks were still common near Fraziers Well until quite recently, so the deep erosion there may simply reflect what was happening near the north flank of the ancient mountain belt, and may not be characteristic of the region as a whole.
In summary, rivers in southern Utah probably flowed northward into a large lake as recently as 40 million years ago. Drainage directions in central Arizona were northward until perhaps as recently as 18 million years ago. We do not know whether these drainages continued into Utah. We do know that throughout this time the present Colorado Plateau had the shape of a gigantic saucer, with the center lower than its uplifted rims, and we know that through most of northern Arizona and southern Utah the strata of the rocky layer cake sloped gently to the northeast, features that were later to have a major role in sculpting the region.
So much for what we know. Little as it is, this knowledge raises a major and intractable problem concerning the establishment of the Colorado River. The River flows generally south, which, in Arizona at least, is away from the low part of the saucer and up over its rim, not what one would normally expect. Even worse, whatever information we have about ancient river systems in the region indicated that they flowed north, in the opposite direction of the present Colorado. This—how to go from rivers flowing north to a river flowing south—is the major problem in the history of the Colorado River, not the carving of the Grand Canyon. Making sense of the problem is not helped by the fact that most, and perhaps all, the critical evidence is gone, destroyed by the endless working of erosion, which has patiently fretted away thousands of feet of rock, lowering the landscape so much that the level where it used to be when the critical action took place so long ago, is now up there, in thin air, visible only in the mind's eye, if at all.
Dr. Ivo Lucchitta
This is the fifth in a series of “Letters from Grand Canyon” by Ivo Lucchitta that will appear in future issues of the bqr.